Plane & Pilot
Saturday, December 1, 2007

Catastrophic Structural Failure


Focusing on maintenance programs


The overwhelming majority of airplanes have the potential to keep flying until it’s no longer economically viable to keep them in the air, provided that they’re operated within established parameters, receive regular inspections to detect problems and undergo proper preventive maintenance. When there’s a catastrophic structural failure, such as a wing falling off, it understandably attracts attention from the industry, investigators and regulators. The failure often can be traced to actions of the pilot, which caused the design stress limits of the airplane to be exceeded, such as flying into a thunderstorm. Much less frequently, the failure is found to have been progressive in nature, culminating in the propagation of a fatigue crack, which compromised structural integrity. Catastrophic structural failures don’t usually attract much attention outside of the aviation community, the exception being when the aircraft is in commercial service and carrying a load of passengers.

One accident that failed to make the front pages occurred on May 9, 2005, when a student pilot and instructor were flying in the Kissimmee, Fla., area for a half-hour aerobatics lesson. The instructor had been demonstrating various maneuvers before the North American SNJ-6 airplane was seen entering a spin, descending rapidly and hitting the ground. Both occupants were killed. The airplane’s right wing had separated in flight. Investigators discovered that the outboard right-wing lower attachment bracket had failed because of fatigue cracking, resulting in separation of the wing. The FAA issued an Emergency Airworthiness Directive requiring fluorescent-dye-penetrant inspections of the wing-attach flanges at 200-hour intervals.

At the other end of the attention spectrum was the December 19, 2005, accident involving Chalk’s Ocean Airways flight 101, a Grumman Turbo Mallard amphibian. It had taken off from the Miami Seaplane Base in Florida on a regularly scheduled passenger flight to Bimini in the Bahamas. There were two pilots and 18 passengers on board. The seaplane base is on an island about two miles east of Miami. The water runway area is 15,000 feet long and 200 feet wide. The terminal on the island used by Chalk’s Ocean Airways was built in 1926.

About one minute after takeoff, the right wing separated, and the airplane crashed into a shipping channel adjacent to the Port of Miami. All on board were killed. The NTSB recently completed its investigation into the accident, which paid a lot of attention to the age and specialized nature of the aircraft, the operator’s maintenance programs and practices, and FAA oversight.

Things being the way they are in the world today, one of the first issues addressed was whether the crash involved terrorism or some other form of sabotage. A video taken by a witness who was on a beach showed the airplane in a nose-down attitude of between 35 and 45 degrees after the wing separated. It didn’t show smoke or debris coming from the airplane, but did show a cloud of fire and smoke behind it. A video taken by a U.S. Coast Guard surveillance camera showed the airplane passing over Miami Harbor and moving away from the camera’s position. As the airplane moved away, it also moved toward the center of the camera’s field of view and became progressively smaller. About nine seconds after the airplane had gone too far away for the camera to pick up, a bright flash appeared. Black smoke could be seen in the area where the flash appeared, and a smoke trail fell toward the water. Pieces of the recovered wreckage were sent to the FBI’s laboratory in Quantico, Va., and were screened for explosives and explosive residue. None were found.




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